Rechargeable lithium batteries are used especially in portable electronic equipment such as telephones, computers and video equipment and recently also in vehicles such as electric bicycles and cars. These applications place high demands on these batteries. In particular they should store the maximum amount of energy for a given volume or weight. They should also be reliable and environmentally-compatible. High energy density and high specific energy are thus two basic requirements which are placed in particular on the electrode material of such batteries.
A further important requirement for such electrode material is resistance to cycling. Here each cycle comprises one charging and discharge process. The resistance to cycling substantially determines the specific charge which is available after several cycles. Even with an assumed resistance to cycling of 99% in every cycle, the available specific charge after 100 cycles would be only 37% of the initial value. Even such a comparatively high value of 99% is therefore largely insufficient. A suitable rechargeable high-performance battery of the type described above should therefore be able not only to store a specific amount of energy at the lowest possible weight and volume, but should also have the ability to discharge and recharge this energy several hundred times. The critical factor here is to a large extent the electrode material.
On account of the major economic importance of such batteries, great efforts have been made to find electrode materials which meet the aforementioned requirements to the maximum extent.
To date, the materials used for the positive electrode of rechargeable lithium batteries have been in particular transition-metal oxides or transition-metal sulphides, organic molecules and polymers. In particular the transition-metal oxides and sulphides have proved successful in practice. Such materials are described as insertion electrode materials and are found in many batteries which are rechargeable at room temperature. The reason for the wider distribution of such materials lies in the fact that the electrochemical insertion reactions are topochemical and thus partially structure preserving.
The idea of a rechargeable battery based on lithium insertion reactions was developed in the 1970s. In the meantime, numerous electrodes based on this principle have been proposed and implemented. The rechargeability of lithium cells is based mainly on the dimensional stability of the guest material during the insertion and removal of Li+.
As referred to above, several transition metal oxides, sulfides, phosphates and halogenides are known as easily reversible materials for positive electrodes. They include in particular lithium cobalt oxides, lithium nickel oxides, lithium manganese oxides, and lithium vanadium oxides, copper oxyphosphate, copper sulphide, lead sulphide and copper sulphide, iron sulphide, copper chloride etc. These materials are however to some extent unsuitable. Thus for example the lithium cobalt oxides are relatively expensive and not especially environmentally compatible. From the standpoint of environmental compatibility, the lithium manganese oxides would be particularly suitable. It has however been found that these oxides generally have a spinel structure which results in them having a lower specific charge or being less stable under cycling with respect to lithium exchange. Tests have also shown that, with the removal of lithium, orthorhombic lithium manganese oxide takes on a spinel structure. With regard to the prior art, reference is made here to the publication “Insertion Electrode Materials for Rechargeable Lithium Batteries” by Martin Winter, Jürgen O. Besenhard, Michael E. Sparh and Petr Novák in ADVANCED MATERIALS 1998, 10 Nov. no. 10, pages 725 to 763, and to dissertation ETH no. 12281 by M. E. Spahr, “Synthese und Charakterisierung neuartiger Oxide, Kohlenstoffverbindungen, Silicide sowie nanostrukturierter Materialien und deren elektro-und magnetochemische Untersuchung” (“Synthesis and characterization of new types of oxides, carbon compounds, silicides and nano-structured materials and their electro- and magneto-chemical analysis.”).
Thus, there is still a great need for improved batteries, especially in terms of high specific energy and large power density.